The present invention relates generally to magnetic resonance imaging (MRI), and more particularly to a method and apparatus for efficient MRI tissue differentiation using an RF pulse designed to provide a frequency response combining a magnetization transfer contrast and fat suppression simultaneously.
Coronary artery disease is currently the leading cause of death in western hemisphere countries. Therefore, the visualization of the coronary arteries is an important step in preventing deaths due to coronary artery disease. Currently, coronary artery visualization is accomplished using x-ray contrast angiography, a highly invasive procedure. In x-ray angiography, a catheter is inserted into the artery through the groin area of a patient in order to accomplish x-ray angiography. It would be advantageous to acquire angiographic images of the coronary arteries without having to require such an invasive procedure.
Magnetic resonance angiography (MRA) is a non-invasive alternative to x-ray angiography that has been successfully used to image carotid arteries, the aorta, and other peripheral vessels. However, coronary arteries are small, tortuous vessels, usually no more than 1-5 mm. in diameter. Successful visualization of coronary arteries using conventional MRA is hampered because coronary arteries are often surrounded by pericardial fat and myocardium, especially the more distal segments. Under known methods, a high degree of fat suppression can be achieved using standard chemical saturation pulses. However, suppression of the myocardial signal, without an accompanying reduction in the blood signal, has heretofore been a challenging problem. Since the myocardial signal is a significant attribute to the contrast-to-noise ratio (CNR) of the coronary arteries, suppression of myocardium is essential for reliable visualization of more complete portions of the left anterior descending (LAD) coronary arteries, as well as the distal segments of the right coronary artery.
A technique known as magnetization transfer contrast (MTC) has been commonly employed in imaging sequences to improve image contrast but has heretofore been employed only in limited manner in MRA of the coronary arteries because of various limitations. The MTC technique relies on the fact that a fraction of the water in biological systems is bound to large macromolecules. This results in lowering of their tumbling rates and consequently, their relaxation times. The "bound pool" is in chemical exchange with the "free pool", therefore resulting in transfer of magnetization. Saturation of the bound pool will then result in saturation of the free pool. MTC effects have been found to adequately suppress myocardial signals in human and animal hearts and can be produced by using either a binomial, zero-degree on-resonance excitation, or an off-resonance spectrally selective excitation. In the past, the off-resonance irradiation utilizes RF pulses of high B.sub.1 amplitude and are set to at least 1 kHz from the water resonance frequency. The large resonance offset avoids undesired saturation of spins with long T.sub.2 times in the imaged volume. However, the further off-resonance the irradiation is, the higher the RF power level is required for observable MTC effects. Continuous wave MTC results in too high of a specific absorption rate (SAR) to the patient, and requires an ancillary RF amplifier on conventional whole-body MR imaging scanners, and is therefore not widely used or desirable.
The off-resonant MTC technique is most effectively achieved using a train of Gaussian pulses of fixed bandwidth, for example, 500 Hz, at 1500 Hz off-resonance. The entire duration of the MTC pulse composite is typically about 200-300 ms. In MRA using a cardiac-gated 3D fast-gradient recalled echo sequence, several problems arise. First, the specific absorption rate of these pulses is very high, thereby limiting its use. It also prohibits use of continuous RF excitation, which is another mechanism for suppressing myocardial signals that can be used in conjunction with magnetization transfer based methods. However, when such RF pulses are applied at a time-delay from the start of the R-wave in the R--R interval, until mid-diastole, the resulting pulses are quite long and consequently not practical for use in interleaved or segmented 3D fast gradient recalled echo sequence, hybrid sequences, where multiple k-space lines are acquired in the same R--R interval, or any other fast image acquisition sequence.
It would therefore be desirable to have a method and apparatus capable of imaging coronary arteries with a lower effective specific absorption rate using the advantages of the MTC effect without the disadvantages while simultaneously suppressing fat tissue to provide enhanced visibility of tissue where the contrast between blood and the surrounding tissue is typically poor.